Method and System For Command Interface Protection To Achieve a Secure Interface

ABSTRACT

Aspects of a method and system for command interface protection to achieve a secure interface are provided. A host device may encrypt a command based on a key index generated within the host device, a host device key, a command count, a random number from a slave device, at least one host control word, and a host variable value. The encrypted command may be communicated to the slave device where it may be decrypted based on the key index, the host device key decrypted from a slave device key generated by the slave device, the command count, the random number, at least one slave control word, and a slave variable value. The key index may be utilized in the host and slave devices to select a master key from a key table from which generational derivatives may be generated for command encryption and decryption respectively.

CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE

This patent application makes reference to, claims priority to and claims benefit from U.S. Provisional Patent Application Ser. No. 60/798,943 (Attorney Docket No. 17367US01), which was filed on May 9, 2006.

This application also makes reference to:

U.S. application Ser. No. ______ (Attorney Docket No. 17637US02) filed on even date herewith; and U.S. application Ser. No. ______ (Attorney Docket No. 17712US02) filed on even date herewith.

Each of the above stated applications is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

Certain embodiments of the invention relate to securing information in multimedia systems. More specifically, certain embodiments of the invention relate to a method and system for command interface protection to achieve a secure interface.

BACKGROUND OF THE INVENTION

In a multimedia system, such as a set-top box (STB), for example where multiple chips on a board communicate with each other, the security of the interfaces between these chips have to be considered in determining the overall security provided by the system. In particular, when one chip is a master device and another chip is a slave device, and when the communication between the master and slave devices occurs over an easily tapped or hijacked medium such as the PCI bus, for example, steps may need to be taken to ensure that the slave device may not be controlled by an unauthorized device. This may be especially important when, for example, the master or host device may be responsible for providing network conditional access (CA) decryption on video/audio data received by the multimedia system, and then re-encrypting the original data using a local copy-protection (CP) method before communicating the re-encrypted data to the slave device. In this instance, an unauthorized user may recover the video/audio data when the data communication between the host and slave devices is compromised. In this regard, an unauthorized user may attack the slave device instead of the higher security host device to recover the original video/audio data with the CA encryption removed.

In master-slave device configurations on a multimedia system board, for example, often the slave device has limited processing capabilities and may not be capable of providing a full software layer. In these instances, the host device may be utilized to configure the slave device via a plurality of commands sent over a generic interface, such as the PCI bus, for example. This may result in an additional level of exposure to unauthorized users since the slave device may not be capable of distinguishing when commands are being sent from the authorized host device or from an unauthorized host device. Without being able to make this distinction, the slave device may be configured by another device on the PCI bus, for example, regardless of the device's authorization status.

Authentication generally refers to the method by which the slave device may ensure that a command being sent to it comes from a legitimate or authorized host device. A slave device may be enabled to accept command from a single authorized device. A generally utilized approach for allowing the slave device to authenticate the host device is to use some sort of public key/private key protocol, where host commands may be signed using its private key, and which are then authenticated by the slave device using the public key. However, without a common shared secret, this type of authentication may be subject to a “man in the middle” attack, for example. In such an attack, a device on the common bus may write a public key to the slave device, where the public key may corresponds to the unauthorized device's own private key, and thus enable the unauthorized device to issue commands that may be considered “authenticated” by the slave device. Another problem with this approach is that the public/private key pair is common for all devices. Therefore, if the private key is ever compromised, it will compromise the devices that utilize this scheme.

Another level of security may include a common secret that may be utilized to provide authentication between devices. In this approach, the host device may encrypt commands that may somehow incorporate the common secret and the slave device may compare this value to its internally stored common secret to ensure that the command originates from an authorized source. However, while a common secret may be useful for helping to keep the details of key exchange secure, exposure of the common secret will compromise the devices.

Another approach may involve the use of a unique per device pair, sometimes known as “pairing”, in order to ensure that for any given slave device there may a single master device. In many STB integrated circuits (ICs), an internal non-volatile memory, such as a one-time-programmable (OTP) or on-chip secure flash, for example, may be utilized to store secure root keys which may be unique for every chip. Utilizing this approach, both the host and slave devices may be programmed with the same secure root key. As a result, commands sent by the host device may be encrypted by, for example, some derivative of the secure root key. Since only a single master device has this key, the slave device may be assured that the command came from the single legitimate source.

However, “pairing” may have logistical and/or practical limitations. In most integrated circuit fabrication and assembly facilities, it may be very difficult to keep track of pairs of chips or devices throughout the manufacturing, validation, testing, and/or board mounting stages. In addition, different types of chips, as the host and slave devices generally are, may be manufactured separately, and a large amount of time and expense may be used to keep track of pairs of devices from two separate fabrication processes and/or sources in order to enable a “pairing” scheme.

Another type of exposure to unauthorized users faced by a slave device may be in the form of replay attacks. For example, when a particular command produces a desired result, an attacker that is tracking the system's activity may remember this command and may later issue the exact sequence in order to achieve the same result. In this regard, effectively reducing or eliminating the ability of unauthorized users to replay authorized command sequences in multimedia systems may enable secure communication interfaces between host and slave devices.

Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.

BRIEF SUMMARY OF THE INVENTION

A system and/or method is provided for command interface protection to achieve a secure interface, substantially as shown in and/or described in connection with at least one of the figures, as set forth more completely in the claims.

These and other advantages, aspects and novel features of the present invention, as well as details of an illustrated embodiment thereof, will be more fully understood from the following description and drawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an exemplary multimedia system, in accordance with an embodiment of the invention.

FIG. 2 is a block diagram illustrating exemplary multimedia system components for generating a common secure key utilizing on-the-fly pairing command authentication, in accordance with an embodiment of the invention.

FIG. 3 is a flow diagram illustrating exemplary steps for generating a common secure key utilizing on-the-fly pairing command authentication, in accordance with an embodiment of the invention.

FIG. 4 is a block diagram illustrating exemplary multimedia system components for decoding a common secure key to enable on-the-fly pairing command authentication, in accordance with an embodiment of the invention.

FIG. 5 is a flow diagram illustrating exemplary steps for decoding a common secure key utilizing on-the-fly pairing command authentication, in accordance with an embodiment of the invention.

FIG. 6 is a block diagram illustrating an exemplary multimedia system that supports on-the-fly pairing for command authentication, key exchange operations, and/or replay attack protection, in accordance with an embodiment of the invention.

FIG. 7 is a flow diagram illustrating exemplary steps for the operation of the multimedia system in FIG. 6, in accordance with an embodiment of the invention.

FIG. 8 is a flow diagram illustrating exemplary steps for extending the replay attack protection of the multimedia system in FIG. 6, in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Certain embodiments of the invention may be found in a method and system for command interface protection to achieve a secure interface. Aspects of the invention may comprise a host device that may enable encryption of a command based on a key index generated within the host device, a host device key, a command count, a random number from a slave device, at least one host control word, and a host variable value. The encrypted command may be communicated to the slave device where it may be decrypted based on the key index, the host device key decrypted from a slave device key generated by the slave device, the command count, the random number, at least one slave control word, and a slave variable value. The key index may be utilized in the host and slave devices to select a master key from a key table from which generational derivatives may be generated for command encryption and decryption respectively.

In an embodiment of the invention, a master device and a slave device may be integrated circuits that may be mounted onto a single circuit board utilized in a set-top-box (STB) operation and/or in other similar applications, for example. The master or host device may be utilized to remove network conditional access (CA) decryption from the incoming data and to re-encrypt the data utilizing a different algorithm such as a local copy-protection (CP) method, for example. The decrypted/re-encrypted data may be communicated to the slave device via a PCI memory transfer and/or a dedicated serial interface, for example. In some instances, a mailbox command protocol may be utilized between the host and the slave devices over a PCI bus, for example, to send command and/or to configure at least a portion of the slave device. In this regard, the communication of data between the master or host device and the slave device may require that the mailbox communication scheme provide a secure exchange, for example, commands on this interface may require encryption and may utilize at least a first level of protection against replay attacks. When processing commands, the slave device may be able to authenticate the source of the commands to ensure that it is a legitimate device. Master keys may be utilized to create final session keys for commands and/or CP data encryption/decryption, for example. In this regard, a secure method of passing the master keys between the host and slave devices may be implemented. Moreover, to protect from rogue devices tapping into or snooping on the common bus, unauthorized devices may not be able to access the slave device's internal registers and/or access the slave device's private memory, such as DRAM memory, for example, to read clear intermediate data.

FIG. 1 is a block diagram illustrating an exemplary multimedia system, in accordance with an embodiment of the invention. Referring to FIG. 1, there is shown a multimedia system 100 that may comprise at least one slave device 102, a memory 110 associated with a slave device 102, a host device 104, a security processor 106, and a bus interface 108. The multimedia system 100 may be utilized in a set-top-box and/or other similar applications, for example. The bus interface 108 may be a PCI bus interface, for example. The slave device 102 may comprise a one-time-programmable memory 112. The host device 104 may comprise a one-time-programmable memory 114. In an exemplary embodiment of the multimedia system 100 shown in FIG. 1, the multimedia system 100 may comprise a plurality of slave devices 102 ₁, . . . , 102 _(K) that may be utilized with associated memories 110 ₁, . . . , 110 _(K), and corresponding one-time-programmable memories 112 ₁, . . . , 112 _(K). The host device 104, the slave devices 102 ₁, . . . 102 _(N), the security processor 106, and/or the memories 110 ₁, . . . , 110 _(K) may be implemented as at least one integrated circuit (IC), for example.

The host device 104 may comprise suitable logic, circuitry, and/or code that may enable control and/or management of operations in the multimedia system 100. For example, the host device 104 may be utilized to generate and send commands to at least a portion of the components in the multimedia system 100 via a bus interface, such as the bus interface 108. In this regard, the host device 104 may also enable secure communication with at least a portion of the components in the multimedia system 100, such as the slave devices 102 ₁, . . . , 102 _(K), by utilizing command authentication schemes over the common bus interface. The host device 104 may support either or both an on-the-fly pairing command authentication operation and an automatic OTP key programming command authentication operation. The host device 104 may also support other secure operations such as key exchange, prevention of replay attacks, and/or prevention of memory attacks, for example. In this regard, the host device 104 may support the use of one or more of the command authentication operations, the key exchange, the prevention of replay attacks, and the prevention of memory attacks, for example, to enable a secure interface between the host device 104 and the slave devices 102 ₁, . . . , 102 _(K). The host OTP memory 114 within the host device 104 may comprise suitable logic, circuitry, and/or code that may enable storage of information that may be utilized for supporting command authentication operations.

The slave devices 102 ₁, . . . , 102 _(K) may each comprise suitable logic, circuitry, and/or code that may enable processing of audio and/or video data in the multimedia system 100. The slave devices 102 ₁, . . . , 102 _(K) may receive commands for processing audio and/or video data and may require some guarantee that the commands received originate from an authorized source. For example, when receiving commands from, for example, the host device 104, the slave devices 102 ₁, . . . , 102 _(K) may support authentication of the received commands based on either or both an on-the-fly pairing command authentication operation and an automatic OTP key programming command authentication operation. The slave devices 102 ₁, . . . , 102 _(K) may also support other secure operations such as key exchange, prevention of replay attacks, and/or prevention of memory attacks, for example. In this regard, the slave devices 102 ₁, . . . , 102 _(K) may support the use of one or more of the command authentication operations, the key exchange, the prevention of replay attacks, and the prevention of memory attacks, for example, to enable a secure interface between the host device 104 and the slave devices 102 ₁, . . . , 102 _(K). The slave OTP memories 112 ₁, . . . , 112 _(K) within a slave devices 102 ₁, . . . , 102 _(K) may comprise suitable logic, circuitry, and/or code that may enable storage of information that may be utilized for supporting command authentication operations.

The memories 110 ₁, . . . , 110 _(K) may comprise suitable logic, circuitry, and/or code that may enable storage of data associated with the operations performed by the slave devices 102 ₁, . . . , 102 _(K). For example, for command authentication operations, the memories 110 ₁, . . . , 110 _(K) may be utilized for storage of data, such as encoding and/or decoding keys, for example, that may be utilized to authenticate that commands received by the slave devices 102 ₁, . . . , 102 _(K) were sent from an authorized source, such as the host device 104. In some instances, the memories 110 ₁, . . . , 110 _(K) may be portions of a single or multiple memory devices, such as a FLASH memory, for example. The memories 110 ₁, . . . , 110 _(K) may be utilized by the slave devices 102 ₁, . . . , 102 _(K) to support other secure operations such as key exchange, prevention of replay attacks, and/or prevention of memory attacks, for example, to enable a secure interface between the host device 104 and the slave devices 102 ₁, . . . , 102 _(K).

The security processor 106 may comprise suitable logic, circuitry, and/or code that may enable providing security operations to the multimedia system 100. For example, the security processor 106 may support command authentication operations to protect the data processed and the operations performed by the slave devices 102 ₁, . . . , 102 _(K) from being accessed by unauthorized sources. In this regard, the security processor 106 may support either or both an on-the-fly pairing command authentication operation and an automatic OTP key programming command authentication operation in the slave devices 102 ₁, . . . , 102 _(K). The security processor 106 may also support security operations that control data and/or command access to at least a portion of the components in the multimedia system 100. In some instances, the security processor 106 may also be utilized to support other secure operations such as key exchange, prevention of replay attacks, and/or prevention of memory attacks, for example, to enable a secure interface between the host device 104 and the slave devices 102 ₁, . . . , 102 _(K).

FIG. 2 is a block diagram illustrating exemplary multimedia system components for generating a common secure key utilizing on-the-fly pairing command authentication, in accordance with an embodiment of the invention. Referring to FIG. 2, there is shown the host device 104, a slave device 102 ₁ and its associated memory 110 ₁. The host device 104 may comprise a host OTP root key (OTPhost) 201 that may be stored within, for example, the host OTP memory 114 as disclosed in FIG. 1. The host device 104 may also utilize a plurality of host control words HCW₁, . . . , HCW_(N), and a plurality of encoders 202 ₁, . . . , 202 _(N). The plurality of encoders 202 ₁, . . . , 202 _(N) may comprise suitable logic, circuitry, and/or code that may enable encryption operations for implementing a key ladder, such as a cipher operated in a recursive manner, for example, that may be utilized for multiple encryption of user-specific host control words with derivatives of the host OTP root key 201 to generate a final derivative value or host key, Khost. The host device 104 may receive at least one signal, which may facilitate generation of the host key, Khost, based on the host OTP root key 201 stored within the host device 104, for example. The host device 104 may also receive at least one signal to pass the host key, Khost, to the slave device 102 ₁, for example.

The slave device 102 ₁ may comprise a slave OTP root key (OTPslave) 203 that may be stored within, for example, the slave OTP memory 112 ₁ as disclosed in FIG. 1. The slave device 102, may also utilize a plurality of slave control words SCW₁, . . . , SCW_(N-1), and a plurality of encoders 204 ₁, . . . , 204 _(N). The plurality of encoders 204 ₁, . . . , 204 _(N) may comprise suitable logic, circuitry, and/or code that may enable a key ladder, such as a cipher operated in a recursive manner, for example, that may be utilized for multiple encryption of user-specific slave control words and the host key, Khost, with derivatives of the slave OTP root key 203 to generate a final derivative value or slave key, Khostslave. The slave device 102 _(a) may receive at least one signal, which may facilitate generation of the slave key, Khostslave, based on the slave OTP root key 203 stored within the slave device 102 ₁ and the passed host key, Khost. The slave device 102 ₁ may also receive at least one signal, which may facilitate passing of the slave key, Khostslave, to the memory 110 ₁ for storage, for example. The memory 110 ₁ may comprise a portion 206 for storage of the slave key, Khostslave, generated by the slave device 102 ₁. Notwithstanding the exemplary embodiment disclosed in FIG. 2, on-the-fly pairing may be implemented between the host device 104 and more than one of the slave devices 102 ₁, . . . , 102 _(K).

The key ladder utilized by the host device 104 and/or the slave device 102 ₁ may allow the OTP root key to encrypt user specified data, such as the control words, which is in turn used as the key for the next stage encryption operation. In this regard, key ladders may have more than one encryption level in order to allow a final key to have a proper crypto-distance from the OTP root key. Moreover, the key ladder may allow the final stage to go to a readable register/memory, so that software may access the final derivative key.

FIG. 3 is a flow diagram illustrating exemplary steps for generating a common secure key utilizing on-the-fly pairing command authentication, in accordance with an embodiment of the invention. Referring to FIG. 3, there is shown a flow diagram 300. In step 304, after start step 302, during manufacturing, assembling, and/or testing of at least a portion of a multimedia system 100, when the integrated circuits corresponding to the host device 104 and the slave devices 102 ₁, . . . , 102 _(K) have been mounted on a circuit board, a special command or signal may be communicated to the host device 104, which may facilitate generation and/or passing of the host key, Khost, to at least one of the slave devices 102 ₁, . . . , 102 _(K). In step 306, the special command or signal enables the host device 104 to generate multi-generational derivatives of the host OTP root key 201, OTPhost, using host control words that may be stored in the host device 104. For example, N derivatives of the OTPhost may be generated by utilizing the host control words HCW₁, . . . , HCW_(N). The final derivative value or host key, Khost, may be generated based on the following expression:

K _(host) =F(OTP _(host) , HCW ₁ , HCW ₂ , . . . , HCW _(N)),

where the function F( ) corresponds to the recursive cipher key ladder implemented utilizing the encoders 202 ₁, . . . , 202 _(N).

In step 308, the final derivative value or host key, Khost, may be routed to a readable register/memory within the host device 104 and may be sent to at least one of the slave devices 102 ₁, . . . , 102 _(K). In step 310, at least one command or signal may be generated to disable the host device 104 from subsequent generations and/or passings of the host key, Khost, to the slave devices 102 ₁, . . . , 102 _(K). In step 312, at least one command or signal may be generated to indicate to at least one of the slave devices 102 ₁, . . . , 102 _(K) to generate corresponding slave keys, Khostslave, based on the slave OTP root key 203, OTPslave, stored within a slave OTP memory. In this regard, N−1 derivatives of the OTPslave may be generated by utilizing the slave control words SCW₁, . . . , SCW_(N−1). The slave control words may be different than the host control words. In step 314, the slave device may generate a final derivative value or slave key, Khostslave, based on the host key, Khost, during a final stage of encryption. The final derivative value or slave key, Khostslave, may be generated based on the following expression:

K _(hostslave) =F(OTP _(slave) , SCW ₁ , SCW ₂ , . . . , SCW _(N−1), Khost),

where the function F( ) corresponds to the recursive cipher key ladder implemented utilizing the encoders 204 ₁, . . . , 204 _(N). In step 316, the slave key, Khostslave, may be written to a corresponding memory, such as the memories 110 ₁, . . . , 110 _(K) that may be FLASH memories, for example. After step 316 the process may proceed to end step 318, for example.

The passing of host key, Khost, to the slave device may not be sufficiently secure, since it may expose the root value. However, by disabling the host device 104 as disclosed in step 310, the passing of the host key, Khost, may be performed once during manufacturing, assembling, and/or testing. In this regard, a command or signal may disable the generation and/or passing of the host key, Khost, by disabling or setting at least one OTP bit in the host OTP memory 114. The value of slave key, Khostslave, in memory may not be altered by an attacker or unauthorized user because commands to the slave devices 102 ₁, . . . , 102 _(K) may not be authenticated.

FIG. 4 is a block diagram illustrating exemplary multimedia system components for decoding a common secure key to enable on-the-fly pairing command authentication, in accordance with an embodiment of the invention. Referring to FIG. 4, there is shown the slave device 102 ₁ and the memory 110 ₁ as disclosed in FIG. 2. The slave device 102, may comprise the slave OTP root key (OTPslave) 203 that may be stored within, for example, the slave OTP memory 112 ₁ as disclosed in FIG. 1, and a plurality of encoders 204 ₁, . . . , 204 _(N−1). The slave device 102 _(i) may also comprise a decoder 402 that may comprise suitable logic, circuitry, and/or code that may enable a key ladder, such as a cipher operated in a recursive manner, for example, that may be utilized for decrypting or decoding the host key, Khost, with derivatives of the slave OTP root key 203 based on the slave key, Khostslave, stored in the portion 206 of the memory 110 ₁. The decoded host key, Khost, may be utilized to decrypt and authenticate host commands that are received by the slave device 102 ₁. Notwithstanding the exemplary embodiment disclosed in FIG. 4, decoding of a common secure key for on-the-fly pairing authentication operations may be performed by more than one of the slave devices 102 ₁, . . . , 102 _(K).

FIG. 5 is a flow diagram illustrating exemplary steps for decoding a common secure key utilizing on-the-fly pairing command authentication, in accordance with an embodiment of the invention. Referring to FIG. 5, there is shown a flow diagram 500. In step 504, after start step 502, when the multimedia system 100 is powered up, a slave device, such as the slave device 102 ₁, for example, may read the slave key, Khostslave, from the corresponding memory, which may be a FLASH memory, for example. In step 506, the slave device may then utilize hidden constants, such as the slave control words SCW₁, SCW₂, . . . , SCW_(N−1), for example, to obtain the host key, Khost, by performing a decryption operation during the final stage of the recursive cipher key ladder. In step 508, commands received from the host device, such as the host device 104, may be authenticated since the host key, Khost, may be known to both the host and slave devices and the host key, Khost, has not been exposed since the passing to the slave device during manufacturing, assembling, and/or testing of at least a portion of a multimedia system 100 as disclosed in FIGS. 3-4.

In addition to command authentication as a mechanism for ensuring secure communication between devices via a common interface, other techniques may also be utilized to further ensure a secure command interface. For example, a key exchange operation and/or a replay attack protection operation may be utilized in addition to on-the-fly pairing command authentication to further secure a command interface between a host device and a slave device.

In a key exchange operation, for example, a key index may be exchanged or communicated between the host device and the slave device. The key index may be utilized to select a common or master key from a key table that may be common to the host device and to the slave device. In this regard, the host and slave devices utilize the key index to obtain a key that may be common to both devices without exposing the actual key value. While exchanging a key index may be vulnerable in instances when a chip containing the appropriate key table is obtained by an unauthorized user, when this approach is combined with a command authentication scheme, such as on-the-fly pairing, for example, and/or with other operations, such as replay attack protection, for example, then the use of a master or common key obtained via the key index may further secure a command interface and the appropriate host device may be authenticated as an authorized source for communicating commands to the slave device.

In a replay attacks, for example, an attacker or unauthorized user may be tracking the commands being sent to the slave device and when a particular command produces a desired result from the slave device, the attacker or unauthorized user may remember the command and may later issue a similar sequence in order to achieve the same result from the slave device. Providing mechanisms to reduce or eliminate the likelihood of a replay attack may also be utilized to further secure a command interface.

FIG. 6 is a block diagram illustrating an exemplary multimedia system that supports on-the-fly pairing for command authentication, key exchange operations, and/or replay attack protection, in accordance with an embodiment of the invention. Referring to FIG. 6, there is shown a host device 104 and a slave device 102 ₁ that may be comprised within the multimedia system 100 disclosed in FIG. 1. The host device 104 may comprise a host key table 602 a and plurality of encoders 604 a. The slave device 1021 may comprise a slave key table 602 b, a plurality of encoders 604 b, and a decoder 606.

The host device 104 may comprise suitable logic, circuitry, and/or code that may enable generation of a key index for selecting a host common or master key from the key table 602 a. The host device 104 may also enable communicating the key index to a slave device, such as the slave device 102 ₁ as disclosed in FIG. 6. The key table 602 a may comprise suitable logic, circuitry, and/or code that may enable storing at least one common or master key that may be utilized by the host device 104 for encrypting a host command to be communicated to the slave device 102 ₁. The key table 602 a may be implemented in at least a portion of a memory integrated within the host device 104.

The encoders 604 a may comprise suitable logic, circuitry, and/or code that may enable encoding or encrypting data based on a key ladder, such as a cipher operated in a recursive manner, for encrypting a host command to be communicated to the slave device 102 ₁. In this regard, an encoder 604 a may be utilized to encode the host key, Khost, generated by the host device 104 based on the host master key selected from the key table 602 a. Another encoder 604 a may be utilized to encode, based on the encoded host key, a control word that results from a combination and/or merging of data that may comprise of a command count, a random number received by the host device 104 from the slave device 102 ₁, at least one host control word stored within a portion of a memory integrated within the host device 104, and a variable value generated by the host device 104. The command count may correspond to a value or number associated with the order and/or sequence in which commands are issued by the host device 104. Another encoder 604 a may be utilized to encode a host command based on the results from encoding the combined and/or merged data. Notwithstanding the embodiment of the invention disclosed in FIG. 6, a plurality of encoders may be utilized to obtain generational derivatives of the master key in order to encrypt or encode the host command to be communicated to the slave device 102 ₁. The host device 104 may enable communicating the encrypted host command to the slave device 102 ₁ via, for example, the bus interface 108 disclosed in FIG. 1.

The slave device 102 ₁ may comprise suitable logic, circuitry, and/or code that may enable receiving a key index communicated from the host device 104 for selecting a slave common or master key from the key table 602 b. The key table 602 b may comprise suitable logic, circuitry, and/or code that may enable storing at least one common or master key that may be utilized by the slave device 102 ₁ for decrypting a host command communicated from the host device 104. The key table 602 b may be implemented in at least a portion of a memory integrated within the slave device 102 ₁. In this regard, the contents and/or indexing of the contents comprised within the key table 602 b may be the same or substantially similar to the contents and/or indexing of the contents comprised within the key table 602 a in the host device 104. The slave device 102 ₁ may also enable generating at least one random number that may be communicated to the host device 104 via, for example, the bust interface 108 disclosed in FIG. 1.

The encoders 604 b may comprise suitable logic, circuitry, and/or code that may enable encoding or encrypting data based on a key ladder, such as a cipher operated in a recursive manner, for decrypting a host command communicated from the host device 104. In this regard, an encoder 604 b may be utilized to encode the host key, Khost, based on the slave master key selected from the key table 602 b. The slave device 102, may generate the host key, Khost, by retrieving from memory and decoding the slave key, Khostslave, generated via an on-the-fly pairing command authentication operation, for example. Another encoder 604 b may be utilized to encode, based on the encoded host key, a control word that results from a combination and/or merging of data that may comprise of a command count, the random number generated by the slave device 102 ₁, at least one slave control word stored within a portion of a memory integrated within the slave device 102 ₁, and a variable value generated by the slave device 102 ₁. The command count may correspond to a value or number kept by the slave device 102 ₁ and associated with the order and/or sequence in which commands are issued by the host device 104. The at least one slave control word and the variable value may correspond to the at least one host control word and variable value utilized by the host device 104 for encrypting the host command.

The decoder 606 may comprise suitable logic, circuitry, and/or code that may enable decrypting the encrypted host command based on the results from encoding the combined and/or merged data. Notwithstanding the embodiment of the invention disclosed in FIG. 6, a plurality of encoders may be utilized to obtain generational derivatives of the master key in order to decrypt or decode the encrypted host command received by the slave device 102 ₁. FIG. 7 is a flow diagram illustrating exemplary steps for the operation of the multimedia system in FIG. 6, in accordance with an embodiment of the invention.

FIG. 7 is a flow diagram illustrating exemplary steps for the operation of the multimedia system in FIG. 6, in accordance with an embodiment of the invention. Referring to FIG. 7, there is shown a flow diagram 700. In step 704, after start step 702, when the multimedia system 100 is powered up, the host device 104 may request a random number from a slave device, such as the slave device 102 ₁, for example. The slave device 102 ₁ may generate the random number and may communicate the generated random number to the host device 104 via a bus interface, such as the bus interface 108, for example. The host device 104 and the slave device 102 ₁ may store the random number in memory, such as memory that may be integrated within the host device 104 and the slave device 102 ₁ respectively, for example.

In step 706, the host device 104 may generate the host key, Khost, based on multi-generational derivatives of the host OTP root key 201, OTPhost, using host control words HCW₁, . . . , HCW_(N) as disclosed in FIGS. 2-3 or may retrieve the host key, Khost, stored in a local memory within the host device 104 when the ability to generate the host key, Khost, has been disabled. In step 708, the slave device 102 ₁ may generate a pre-key in the next to last stage of a recursive cipher key ladder based on the slave OTP root key 203, OTPslave, stored within a slave OTP memory and hidden constants, such as the slave control words SCW₁, . . . , SCW_(N−1). In step 710, the slave device 102 ₁ may read the slave key, Khostslave, from the corresponding memory, which may be a FLASH memory, for example, and may utilize the pre-key generated in step 708 to obtain the host key, Khost, by performing a decryption operation during the final stage of a recursive cipher key ladder.

In step 712, the host device 104 may generate a key index and may communicate the key index to the slave device 102 ₁ via the bus interface 108. In step 714, the host device 104 and the slave device 102 ₁ may each select a master or common key from the key tables 602 a and 602 b, respectively, based on the key index. In step 716, a first level of encryption may be performed in the host device 104 and the slave device 102 ₁ based on the selected master or common keys and on the shared host key, Khost. In step 718, a control word comprising at least a portion of the command count, the random number, at least one control word, and a variable value may be generated in the host device 104 and in the slave device 102 ₁.

In step 720, the control words generated in step 718 may be encrypted in the host device 104 and in the slave device 102 ₁ respectively by utilizing the results from step 716. In step 722, the host device 104 may encrypt the host command based on the results from step 720 generated within the host device 104. The host device 104 may communicate the encrypted host command to the slave device 102 ₁ via the bus interface 108. In step 724, the slave device 102 ₁ may decrypt the encrypted host command based on the results from step 720 generated in the slave device 102 ₁. When the encrypted host command is properly decrypted and understood by the slave device 102 ₁, then the slave device 102 ₁ may authenticate that the command is received from an authorized source. Otherwise, the command received was not from an authorized source. After step 724, the process may proceed to end step 726.

Since the host device 104 and the slave device 102 ₁ utilize the same and/or similar values in their corresponding recursive cipher key ladders, the slave device 102 ₁ may understand the values encrypted by the host device 104. In this regard, the approach described herein may ensure that commands may originate from one authorized host device and be decrypted by the appropriate slave device. Moreover, the exchange of a random number, the random number may be generated by either the host device 104 or by the corresponding slave device being paired with the host device 104. In this regard, the exchange of the random number need not be a secure exchange.

After the multimedia system 100 is powered up, the host device 104 may keep track of a value associated with command count. The value may be incremented when a command or when certain commands are issued or generated by the host device 104. When the host device 104 is ready to generate a command, the host device 104 may add the command count value to the random number in order to generate a control word for the host command encryption process. In some instances, bit interleaving may be utilized when encrypting the host command. The slave device 102 ₁ may also keep track of a value for the command count and may also have access to the random number. After decrypting the host command, the slave device 102 ₁ may accept those commands that have the correct random number and command count value. The slave device 102 ₁ may reject commands that have incorrect random number and command count value. This approach may ensure that when an unauthorized user re-issues a command that was previously generated, the command counter value may not be correct and the slave device 102 ₁ may reject the command.

In some instances, the command count for the host device 104 and for a slave device may not be synchronized. In this regard, the slave device may be enabled for acceptance of host commands in which the command count may be higher than the command count expected by the slave device. This approach may allow the slave device to accept a host command when the slave device missed a previously issued host command and the expected command count by the slave device had a lower value than the command count encrypted within the received host command. However, when the command count received by the slave device has a value that is lower than the expected command count value, then the host command may be rejected as it may be a replay attack.

FIG. 8 is a flow diagram illustrating exemplary steps for extending the replay attack protection of the multimedia system in FIG. 6, in accordance with an embodiment of the invention. Referring to FIG. 8, there is shown a flow diagram 800. In step 804, after start step 802, the host device 104 may send a request for a random number to a paired slave device, such as the slave device 102 ₁, for example, before sending a host command to the slave device. In step 806, the corresponding slave device may generate the random number and may communicate the random number to the host device 104 via a bus interface, such as the bus interface 108, for example. The slave device may also store the generated random number in local memory integrated within the slave device, for example. In step 808, the host device 104 may utilize the received random number when generating a control word utilized in the encryption of the host command. The host device 104 may then communicate the encrypted host command to the slave device. In step 810, the slave device may parse the host command after being decrypted to obtained the random number communicated from the host device 102. When the communicated and stored random numbers are the same, the slave device may accept the host command. After step 810, the process may proceed to end step 812.

Accordingly, the present invention may be realized in hardware, software, or a combination of hardware and software. The present invention may be realized in a centralized fashion in at least one computer system, or in a distributed fashion where different elements are spread across several interconnected computer systems. Any kind of computer system or other apparatus adapted for carrying out the methods described herein is suited. A typical combination of hardware and software may be a general-purpose computer system with a computer program that, when being loaded and executed, controls the computer system such that it carries out the methods described herein.

The present invention may also be embedded in a computer program product, which comprises all the features enabling the implementation of the methods described herein, and which when loaded in a computer system is able to carry out these methods. Computer program in the present context means any expression, in any language, code or notation, of a set of instructions intended to cause a system having an information processing capability to perform a particular function either directly or after either or both of the following: a) conversion to another language, code or notation; b) reproduction in a different material form.

While the present invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the present invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present invention without departing from its scope. Therefore, it is intended that the present invention not be limited to the particular embodiment disclosed, but that the present invention will include all embodiments falling within the scope of the appended claims. 

1. A method for secure communication in a device, the method comprising: decrypting an encrypted host command in a slave device based on at least one or more of the following: a key index communicated from a host device, a host device key decrypted from a slave device key generated by said slave device, a command count, a random number generated by said slave device, at least one slave control word stored within said slave device, and a slave variable value, wherein said encrypted host command is generated in said host device based on at least one or more of the following: said key index, said host device key, said command count, said random number, at least one host control word stored within said host device, and a host variable value.
 2. The method according to claim 1, comprising tracking said command count in said slave device.
 3. The method according to claim 1, comprising tracking said command count in said host device.
 4. The method according to claim 1, comprising selecting a host master key from a host key table within said host device based on said key index.
 5. The method according to claim 4, comprising generating a first generational derivative of said selected host master key based on said host device key.
 6. The method according to claim 5, comprising generating a second generational derivative of said selected host master key based on said generated first generational derivative, said command count, said random number, said least one host control word, and said host variable value.
 7. The method according to claim 1, comprising selecting a slave master key from a slave key table within said slave device based on said key index.
 8. The method according to claim 7, comprising generating a first generational derivative of said selected slave master key based on said host device key.
 9. The method according to claim 8, comprising generating a second generational derivative of said selected slave master key based on said generated first generational derivative, said command count, said random number, said least one slave control word, and said slave variable value.
 10. A machine-readable storage having stored thereon, a computer program having at least one code section for secure communication in a device, the at least one code section being executable by a machine for causing the machine to perform steps comprising: decrypting an encrypted host command in a slave device based on at least one or more of the following: a key index communicated from a host device, a host device key decrypted from a slave device key generated by said slave device, a command count, a random number generated by said slave device, at least one slave control word stored within said slave device, and a slave variable value, wherein said encrypted host command is generated in said host device based on at least one or more of the following: said key index, said host device key, said command count, said random number, at least one host control word stored within said host device, and a host variable value.
 11. The machine-readable storage according to claim 10, comprising code for tracking said command count in said slave device.
 12. The machine-readable storage according to claim 10, comprising code for tracking said command count in said host device.
 13. The machine-readable storage according to claim 10, comprising code for selecting a host master key from a host key table within said host device based on said key index.
 14. The machine-readable storage according to claim 13, comprising code for generating a first generational derivative of said selected host master key based on said host device key.
 15. The machine-readable storage according to claim 14, comprising code for generating a second generational derivative of said selected host master key based on said generated first generational derivative, said command count, said random number, said least one host control word, and said host variable value.
 16. The machine-readable storage according to claim 10, comprising code for selecting a slave master key from a slave key table within said slave device based on said key index.
 17. The machine-readable storage according to claim 16, comprising code for generating a first generational derivative of said selected slave master key based on said host device key.
 18. The machine-readable storage according to claim 17, comprising code for generating a second generational derivative of said selected slave master key based on said generated first generational derivative, said command count, said random number, said least one slave control word, and said slave variable value.
 19. A system for secure communication in a device, the system comprising: a slave device that enables decrypting an encrypted host command based on at least one or more of the following: a key index communicated from a host device, a host device key decrypted from a slave device key generated by said slave device, a command count, a random number generated by said slave device, at least one slave control word stored within said slave device, and a slave variable value, wherein said encrypted host command is generated in said host device based on at least one or more of the following: said key index, said host device key, said command count, said random number, at least one host control word stored within said host device, and a host variable value.
 20. The system according to claim 19, wherein said slave device enables tracking of said command count.
 21. The system according to claim 19, wherein said host device enables tracking said command count.
 22. The system according to claim 19, wherein said host device enables selection of a host master key from a host key table within said host device based on said key index.
 23. The system according to claim 22, wherein said host device enables generation of a first generational derivative of said selected host master key based on said host device key.
 24. The system according to claim 23, wherein said host device enables generation of a second generational derivative of said selected host master key based on said generated first generational derivative, said command count, said random number, said least one host control word, and said host variable value.
 25. The system according to claim 19, wherein said slave device enables selection of a slave master key from a slave key table within said slave device based on said key index.
 26. The system according to claim 25, wherein said slave device enables generation of a first generational derivative of said selected slave master key based on said host device key.
 27. The system according to claim 26, wherein said slave device enables generation of a second generational derivative of said selected slave master key based on said generated first generational derivative, said command count, said random number, said least one slave control word, and said slave variable value. 